During Digital Subscriber Line (DSL) training, two modems on opposite ends of a DSL connection test the DSL line to determine what bit rates can be used for transferring data in both an upstream and downstream direction. DSL training rates and the Signal-to-Noise Ratio (SNR) on a DSL connection is largely affected by the quality of the line in terms of its ability to carry a signal without distortion. For example, the signal on the DSL connection can degrade when the physical wire carrying the signal is subjected to external interference by electromagnetic sources.
Service providers provision DSL connections in either a rate adaptive mode or a fixed rate mode. The rate adaptive mode seeks to maximize the bit rate for a given line. The bit rate is dependent on the quality of the signal achievable on the line. The fixed rate mode forces the connection to a particular rate. If the line conditions are such that particular rate is not possible, the connection is not made, not even at a lower rate.
In both the rate adaptive mode and the fixed rate mode, the DSL modem training involves a Digital Subscriber Line Access Multiplexer (DSLAM) and Customer Premise Equipment (CPE) exchanging information about the quality of the signal received, or Signal-to-Noise Ratio (SNR). For example, a CPE modem may send out a known tone to the DSLAM. The quality of the tone is evaluated to determine the amount of interference in the DSL line.
The DSLAM and the CPE exchange the signal quality information from each other to determine the transmit (Tx) power (transmit gain) and the receive (Rx) power (receive gain or receiver sensitivity). The data rate on the DSL line is proportional to the Tx and Rx gain. However, excessive Tx gain will interfere with the Rx gain, and possibly disrupt other services in the network. Therefore, DSL modem training is conservative when setting gain values.
Although current implementations of rate adaptive mode are supposed to train the DSL modem at an optimum bit-rate, it is not the case in practice. This is because the rate adaptive algorithm attempts to guess the best rate(s) possible before steady-state operations (Showtime). These best rate(s) are then negotiated with the far end of the DSL connection. This algorithm often does not achieve the optimum bit-rate.
In the fixed rate mode, the DSL line may not train if the line conditions cause the SNR to fall below acceptable limits. However, if the SNR is at the boundary of the acceptable limit, there is no way to adjust the Tx or Rx gain in order to achieve a SNR that is acceptable.
There are times when a user may wish to have a better SNR than what the DSL chipset provides. But current DSL systems leave it up to the DSL chipset to determine the best training parameters. The training algorithms used in the DSL chipset are conservative due to a number of factors. For example, the DSL chipset makes guesses as to the best rates achievable in order to achieve quick training time. These guesses may not yield the best results. Also, a DSL chipset vendor has no knowledge of the PSD (Power Spectral Density) mask that is specific to a particular CPE. Using overly aggressive transmit power could violate transmission regulatory restrictions. To prevent transmission violations, the DSL chipsets use conservative gain parameters that may not provide the optimal transmit and receive gain settings.